Renouncing his U.S. citizenship could shield Saverin from much, though not all, of his U.S. tax liability related to his Facebook holdings. Singapore doesn't have a capital gains tax, although it does tax income earned in the city-state as well as some foreign-source income.

According to Reuven Avi-Yonah, head of the international tax program at the University of Michigan, Americans who give up their citizenship still owe a sort of exit tax on the capital gains from their stock portfolios, whether they've sold shares or not. But estimating the value of shares in a private company such as Facebook is, shall we say, an exercise with lots of room for interpretation.

Avi-Yonah told Bloomberg that renouncing citizenship ahead of an IPO is "a very smart idea" from a tax standpoint, since once a company is public, "you can't fool around with the value."

Saverin gave up his U.S. citizenship "around September," his spokesman told Bloomberg. Facebook formally filed for its initial offering earlier this month. Saverin was born in Brazil, then moved to the U.S. in 1992 and became a citizen in 1998.

David Hamilton is the assistant managing editor of CNET News. He has been writing and editing business and tech coverage for about two decades -- the majority of that at the Wall Street Journal in both Tokyo and San Francisco. He is a two-time winner of the Overseas Press Club award and has written for numerous magazines and blogs, including Slate, Science, VentureBeat, CBS Interactive's BNET, California Lawyer and the New Republic.